Light-induced absorbance changes reflecting the photoreduction of a b-type cytochrome were measured in mycelial tissue of Neurospora crassa. The action spectrum for the photoreduction of cytochrome b was essentially the same as the action spectrum for the effect of light on the circadian rhythm of conidiation. The action spectrum indicated that the light was absorbed by a flavin which mediated the reduction of the cytochrome. Light-induced absorbance changes will also be measured in cell free extracts of the Neurospora crassa mycelium to identify and study the photoreceptor pigment in greater detail. Since the reaction involves the flavin mediated photoreduction of a cytochrome, the reaction will be studied in artificial systems comprised of solutions of flavin and cytochrome. BIBLIOGRAPHIC REFERENCES: Photoreceptor pigment for blue light in Neurospora crassa (1975) Victor Munoz and Warren L. Butler, Plant Physiol. 55:421-426.